Several publications are referenced in this application by numerals in brackets in order to more fully describe the state of the art to which this invention pertains. The disclosure of each of these publications is incorporated by reference herein.
The observation that vasoactive intestinal peptide D (VIP) is cleaved by Abs from asthma patients provided early evidence that Abs may possess peptidase activity [1,2]. This observation has been reproduced independently by Suzuki et al [3]. Autoantibody catalysis is not restricted to catalysis of VIP. Autoantibodies in Hashimoto's thyroiditis catalyze the cleavage of thyroglobulin [4]. Further evidence for autoantibody catalysis has been provided by reports of DNase activity in Abs from lupus patients [5,6]. The bias towards catalytic Ab synthesis in autoimmune disease is supported by observations that mouse strains with a genetic predisposition to autoimmune disease produce esterase Abs at higher levels when compared to control mouse strains in response to immunization with a transition state analog [7].
Like noncatalytic Abs, peptidase Abs are capable of binding Ags with high specificity mediated by contacts at residues from the VL and VH domains. The purified H and L subunits are known to be independently capable of binding Ags, albeit with lower affinity than the parent Ab. X-ray crystallography of Ab-Ag complexes have shown that the VL and VH domains are both involved in binding the Ag [8]. The precise contribution of the two V domains varies in individual Ab-Ag complexes, but the VH domain may contribute at a somewhat greater level, because CDRH3 tends to be longer and more variable in sequence compared to CDRL3.
The initial complexation of a polypeptide Ag by a peptidase Ab is followed by cleavage of one or more peptide bonds. Just prior to cleavage, contacts with the catalytic residues of the antibody are established with the peptide bond in the transition state. The ability to hydrolyze peptide bonds appears to reside in the VL domain. This conclusion is based on the cleavage of VIP by polyclonal autoantibody L chains, monoclonal L chains isolated from multiple myeloma patients and their recombinant VL domains, and recombinant L chains raised by immunization with VIP. The H chains of polyclonal and monoclonal Abs to VIP are capable of VIP binding but are devoid of the catalytic activity [9]. The VH domain can nevertheless influence the peptidase activity by "remote control", because in binding to VIP remote from the cleavage site, it can influence the conformation of the binding site as shown by the peptidase activity of F.sub.v constructs composed of the catalytic anti-VIP VL domain linked to its VH domain. The anti-VIP VH domain exerted beneficial effects and an irrelevant VH domain exerted detrimental effects on the catalytic activity, as evaluated by the values of VIP binding affinity and catalytic efficiency. The proposed existence of distinct catalytic and antigen binding subsites in catalytic Abs is consistent with data that Abs generally contain large combining sites, capable of accommodating 15-22 amino acids of polypeptide substrates [8], and that substrate regions distant from the cleavage site are recognized by the Abs. Thus, the VH domain offers a means to control the specificity of the catalytic site.
Molecular modeling of the L chain suggested that its Asp1, Ser27a and His93 are appropriately positioned to serve as the catalytic triad [10]. The hydrolysis of VIP was reduced by &gt;90% by substitution of Ala residues for Ser27a, His93 or Asp1 by site-directed mutagenesis [12]. The catalytic activity of the wild type protein was inhibited selectively by diisopropylfluorophosphate (DFP), a serine protease inhibitor, but the residual activity of the Ser27a mutant was refractory to DFP. The K.sub.m of the wild type L chain for VIP (130 nM) was unaffected by mutations at Ser27a, His93 and Asp1. In contrast, mutagenesis at residues forming the extended active site of the L chain (Ser26, H27d/Asp28) produced increases in the K.sub.m values (by 10-fold) and increases in turnover (by 10-fold). These results can be explained as arising from diminished ground state stabilization. The consequent decrease of .DELTA.G.sup.+.sub.cat produces an increase in turnover. Thus, two types of residues participating in catalysis by the L chain have been identified. Ser27a and His93 are essential for catalysis but not for initial high affinity complexation with the ground state of VIP. Ser26 and His27d/Asp28 participate in VIP ground state binding and limit turnover indirectly. See FIG. 1.
The VIPase L chain displayed burst kinetics in the early phase of the reaction, suggesting the formation of a covalent acyl-L chain intermediate, as occurs during peptide bond cleavage by serine proteases. The fluorescence intensity was monitored as a function of time after mixing the L chain with the substrate Pro-Phe-Arg-MCA. There was an immediate increase in fluorescence, corresponding to formation of the covalent intermediate, followed by a slower increase, corresponding to establishment of the steady rate. The number of active sites was computed from the magnitude of the burst by comparison with the fluorescence yield of standard aminomethylcoumarin. The concentration of catalytic sites was estimated at 114 nM, representing about 90% of the L chain concentration estimated by the Bradford method (125 nM).
The catalytic residues (Ser27a, His93, Asp1) in the anti-VIP VL domain are also present in its germline VL domain counterpart (GenBank accession number of the germline VL gene, Z72384). The anti-VIP VL domain contains 4 amino acid replacements compared to its germline sequence. These are His27d:Asp, Thr28e:Ser, Ile34:Asn and Gln96:Trp. The germline configuration protein of the anti-VIP L chain was constructed by introducing the required 4 mutations as described previously [12]. The purified germline protein expressed catalytic activity as detected by cleavage of the Pro-Phe-Arg-MCA substrate at about 3.5 fold lower level than the mature L chain (330.+-.23 FU/0.4 .mu.M L chain/20 min; substrate conc. 50 .mu.M). The data suggest that remote effects due to the somatically mutated residues are not essential for expression of the catalytic activity.
The present invention provides novel compositions and methods for stimulating production of catalytic antibodies and fragments thereof. Catalytic antibodies with specificity for predetermined disease-associated antigens provide a valuable therapeutic tool for clinical use. Provided herein are methods for identifying, isolating and refining naturally occurring catalytic antibodies for the treatment of a variety of medical diseases and disorders, including but not limited to infectious, autoimmune and neoplastic disease. Such catalytic antibodies will also have applications in the fields of veterinary medicine, industrial and clinical research and dermatology.